Field of the Invention
The present invention relates to communication systems and, more specifically but not exclusively, to antenna arrays, such as those for cellular communication systems.
Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
An active antenna comprises an array of radiating elements or sub-arrays of radiating elements that are excited in a particular set of relative amplitude and phase excitations to create a desired radiation pattern. For an active array having a column of elements (or sub-arrays), parameters such as downtilt angle, beamwidth, and sidelobe levels can be adjusted by modifying the amplitude and phase excitations at the sub-array level. The relative excitations are controlled by amplifiers, electronic phase shifters, and digital radios at each sub-array or element.
A calibration process is performed to define the response of one transceiver chain relative to the others in order to establish a baseline reference between the elements. Since this reference may change over time due to temperature, drift, or other phenomenon, the calibration process should be easy to use and able to be repeated as needed during the lifetime of the product. The passive components of the calibration process should be time invariant. Low cost and simplicity of implementation are other desired features. Calibration should be applied independently to the transmit path on the downlink and to the receive path on the uplink.
A typical calibration circuit might consist of a directional coupler at each element or sub-array level, connected via interconnects to an n-way splitter/combiner network that combines the coupled signals to a common calibration port. This method has the disadvantage of requiring additional couplers, power dividers, cables, and interconnects with preferably time-invariant responses to transport the signal to the calibration transceiver, all of which add complexity and cost.